Plaster of paris product and process improvements



United States Patent 3,282,265 PLASTER 0F FARE PRUDUCT AND PRUCESS IMPROVEMENTS David F. Smith, 120 Grove St., Bay Head, NJ. No Drawing. Filed Feb. 10, 1965, Ser. No. 431,722 7 Claims. (Cl. 1289 1) This application is a continuation-in-part of application Serial No. 198,444, filed May 29, 1962, and application Serial No. 686,283, filed September 26, 1957, both now abandoned.

This invention relates to new plaster of Paris products and methods of making them. It is particularly related to the so-called plaster of Paris bandages which are made by mixing powdered plaster into a slurry with a volatile liquid in which the plaster does not set during processing, spreading or impregnating the slurry onto an inert, porous, flexible backing material or carrier web and then heating to evaporate the liquid and leave a dry, rapidly water-wettable, settable coated or impregnated web or plaster of Paris bandage. The slurry contains a bonding agent or binder to adhere the plaster particles to themselves and to the backing, and a material such as potassium sulfate to speed or accelerate the set of the plaster when the bandage is subsequently wet in water preparatory to making a cast. Such bandages are usually slit into strips and supplied in strips or rolls for use by ortho pedic surgeons who wet them by immersion in a pail of water, squeeze out the excess water and wrap them around parts of the human or animal body to form a rigid cast which provides needed immobilization and/or suport.

p Such bandages should desirably be bonded so that they do not lose excessive amounts of plaster when handled in the dry state or when wet and squeezed out to make a cast and when the layers of wet bandage are laminated in forming the cast. Even in a roll containing yards of 6 or 8 inch wide bandage, the wetting should be complete throughout in 5 to seconds immersion in water. The wetbandage should set in from about 2 to 8 minutes and make a strong cast A2 to 1 hour after wetting. Specifications and test procedures are given in Federal Specification GG-B-lOld and in Military Medical Purchase Description No. 9, Sept. 26, 1963. The backing material is usually 32 x 28 mesh surgical gauze or crinoline made therefrom. The coating weight is such as to yield a dry, finished 4 inch by 5 yard bandage weighing from 205 to 255 grams when using 32 x 28 mesh gauze which weighs about 8 pounds per 1000 sq. ft.

As slurry liquid, there can be used an inert organic liquid such as methanol, ethanol, n-propanol, i-propanol or acetone either pure or diluted with up to 40% water, or 90 to 95% aqueous acetic acid. There has been used also water containing a small amount of ammonium borate (US. Patent 2,557,083) made by adding aqueous ammonia to boric acid, which retards the set of the plaster during processing but which decomposes during heating 7 and drying of the slurry-coated web to leave only traces of the borate which have only a relatively small effect in slowing the set of the finished bandage. However, even traces of the retarder can appreciably slow the set when it is realized that the bandge must set in a very few minutes; and, due to variations in heating and drying the slurry-coated web, the set of the bandage at best can show variations of a few minutes. This is increasingly true the more set-inhibitor is used relative to plaster. Furthermore, boric acid and its salts are known to have toxic effects when covering a large area of the human body.

While the required delay of the set during processing will depend upon how promptly the slurry is used, for

trin or mixtures of these.

example whether a continuous or batch process is used; in any case it is desirable to have a slurry that will stand for several hours without noticeable setting of the plaster if this can be accomplished without using excessive amounts of inhibitor, since there is normally, of course, at least a minor amount of setting going on during holding of the slurry and the longer the slurry life (pot life) the less the setting during a given period. The pot life is related to the concentration of set-inhibitor in the slurry liquid while the slowing of the set of the finished bandage due to traces of set-inhibitor left therein is less thesmaller the proportion of set-inhibitor to plaster in the slurry. The concentration of set-inhibitor in the slurry is related to the proportion of set-inhibitor to plaster by the proportion of liquid in the slurry to plaster in the slurry. The thickness (viscosity) of the slurry depends, among other things, upon the proportions of liquid to plaster and the thickness must be suited to the method of coating-whether by single roll, double roll, reverse roll or knife coater, etc. A reverse roll coater uses a thin slurry while a knife coater can use a thick slurry. The objects of the present invention are (l) to provide an aqueous slurry liquid that more effectively retards the set during processing, (2) to obtain a dry, finished bandage with a minimum of set-retardation which makes a cast of high early strength with low plaster loss, (3) to obtain a bandage which when wet for use has a smooth consistency and laminates well layer to layer and to parts of the cast that are already set, in contrast to bandages using a hydrophobic binder like polyvinyl acetate which do not laminate well and have a harsh feel, (4) to produce a bandage that when wet for use does not absorb excessive amounts of water so as to make a sloppy bandage and a heavy cast that requires more drying, (5) to make a bandage whose small loss in the immersion pail settles well so as to readily accumulate on the bottom of the pail or in a plaster trap so as to permit reducing possible clogging of waste pipes, (6) make a bandage that has no toxic or allergenic properties on human skin.

Other objects will appear in the following.

The methods of the present invention are illustrated but not limited by the following example.

Example 1 Steam calcined plaster of Paris (alpha-hemihydrate of CaSO lbs 150 Casein (Argentina acid casein), dissolved in part of the aqua ammonia below g 150 Aqua ammonia (28-30% NH total including that used to dissolve the casein) g 16,545 Water (exclusive of that in the aqua ammonia) g 20,220 Potassium sulfate (ground to pass mesh screen) g 1,360

Example 1 shows a preferred slurry that was spread with a roll coater on crinoline, dried for 34 minutes in a circulating air oven at an air temperature of about 200 F., the bandage meeting the above stated objectives. The life of the slurry was over 22 hours. The ammonium caseinate gives a bonding action on the finished bandage as Well as acting in conjunction with the ammonia to prevent setting of the slurry. Additional binder can be added to the slurry in the form of 0.2 to 2% of the Weight of plaster, of methyl cellulose, cooked starch, dex- Corn, tapioca or potato starch may be used or dextrins made from these starches. The potassium sulfate may vary from 0.5 to 2.5 or 3% of the weight of the plaster. Dried and ground gypsum freshly made from plaster or terra alba may also be used as set-accelerator in amount from about 0.2 to 0.5% of the plaster, either in the slurry or, better, on the dry bandage. The proportion of casein (dissolved as ammonium caseinate) may vary from 0.1 to 1% of the weight of the slurry liquid and from about 0.1 to 1% of the plaster, a preferred amount being from 0.15 to 0.3% of the plaster. The NH content of the slurry liquid may vary from about 4 to 20% of the weight of the liquid, a preferred amount being 8 to In Example 1, the percent casein on the Weight of plaster is 0.22 and on the weight of slurry liquid is 0.41, and the percent NH is 13.2 based on the Weight of slurry liquid. A preferred slurry formula contains 100 parts plaster, 0.3 to 0.6 part cooked starch, 0.5 to 1 part dextrin, 0.2 to 0.4 parts methyl cellulose, 0.15 to 0.3 part casein, 4.4 to 10 parts NH 40 to 55 parts Water, 0.03 to 0.05 part Alkanol B and 1.5 to 2.5 parts K 50 This formula gives a satisfactory pot life, a bandage with low dry and wet plaster loss, a wet bandage with a smooth, plastic consistency that laminates well, good settling of waste in the immersion pail and an attractive cast with consistently fast set, high early strength and low weight. The casein appears to have an action on the starch, dextrin and methyl cellulosepresumably between its acid groups and the residual hydroxyl groups of the additional bindersas the slurry is heated and dried. The cooked starch is made by steaming a dilute suspension (not over about 12% in water) at atmospheric pressure for about 1 /2 hrs. The casein is used in the form of fine granules, which are stirred in water until wet, part of the aqua ammonia is then added with continuous stirring for about 10 minutes or until completely dissolved to an only slightly milky solution. This may be accomplished using about of the total water and about 10% of the total aqua ammonia. The casein solution is preferably used when fresh and should not be heated since prolonged heating degrades it to form a gelatinous product. The dex-trin is dissolved in Water by heating to about 70 to 80 C. It is also preferred to use in the slurry from 0.03 to not over 0.1% of the weight of plaster, of a wetting-agent which may be sodium alkyl naphthyl sulfonate (for example, Al kanol B made by E. I. du Pont de Nemours & Co., Wilmington, Del.), sodium lauryl sulfate or other sodium alkylated aromatic mononuclear hydrocarbon sulfonate. The wetting-agent not only improves the Wetting of the dry bandage but also improves the coating of the guaze and gives the bandage a smooth feel, makes it more flexible, less subject to cracking and dusting during slitting. The proportions of slurry liquid to plaster will depend, as stated, on the viscosity best suited to the method of coating, on the type of plaster used and also on the amount and type of additional bonding agent used and, to some extent, on the proportions of ammonia used. By adjustment of these variables, the slurry viscosity can be controlled. In general the amount of slurry liquid can Vary from abou lbs. to about 75 lbs. per 100 lbs. plaster and can be readily determined for any given circumstances, the important variables from the standpoint of this invention being the concentrations of NH and ammonium caseinate in the slurry liquid and the proportion of ammonium caseinate to plaster. The methyl cellulose used can vary in viscosity (in 2% aqueous solution at 20 C.) from 50 to 4000 centipoises and in methoxyl content from 27 to 33%; or hydroxypropyl methyl cellulose of similar viscosity can be used with 19 to 30% methoxyl and 3 to 12% propoxyl content; or hydroxyethyl ethyl cellulose of similar viscosity and degree of substitution of ethoxy and of ethyl groups from 0.05 to 1 per anhydroglucose unit. A preferred procedure is to mix the casein, starch, dextrin, methyl cellulose and A1- kanol B solutions with the amount of water to give the required total. The ammonia required to give the desired total is then mixed in. Then the plaster is mixed in until thoroughly Wet by the solutions. Then the ground potassium sulfate is added and the slurry thoroughly mixed with a high-speed stirrer until the slurry thins down and smooths out.

Applicant has discovered that ammonium caseinate is a much more effective plaster set-inhibitor than ammonium borate. Thus a solution 0.7% in boric acid, 1% in Nl-l and 2% in K 50 delays the set of plaster for 4 /2 hrs. while a solution 0.7% in casein, 1% in NH and 2% in K 50 delay the set of another sample of the same plaster for over 26 hrs. Furthermore, applicant has discovered that a synergistic action on set-inhibition occurs between ammonium caseinate and ammonia. Thus, a solution containing cooked starch and K 0.7% casein and 1.5% NH (only a moderate excess over that needed to dissolve the casein and supress hydrolysis of the ammonium caseinate) delays the set for 2 /2 hrs., while increasing the NH to 8% in the same solution delays the set for 25 hrs. The same solution containing 8% NH Without the ammonium caseinate delays the set for only about /2 hr. A solution containing less than about 4% NH alone has no appreciable effect on the set. This permits using minimal amounts of ammonium caseinate and/ or ammonia. Particularly it permits using a minimal proportion of caseinate to plaster and still protecting the partially dry Web against the possibility of low temperature areas in the drier and minimizes the proportion of caseinate that might be left unhydrolyzed in the dry bandage. High concentrations of ammonia can be used since traces have no appreciable effect on setting. WhileoOncentrated ammonia alone causes some delay of the set, it is highly desirable to have at least a minimal amount of caseinate in the slurry liquid since (1) it has a very desirable action in being strongly chemically adsorbed or reacting with other binders present such as cooked starch, dextrin and cellulose ethers, leaving in the dry bandage a thin coating of the water-insoluble but readily Water-wettable casein after the ammonia has been hydrolyzed therefrom and evaporated; this action decreasing the water solubility and Water sensitivity of these other bonding agents and increasing the settling rate of the plaster lost in the immersion pail-such action being impossible when using a hydrophobic binder like polyvinyl acetate, (2) if failure of the drier occurs or localized cold areas exist therein for any reason, after partial drying the slurry-coated web, it is found that the ready evaporation of ammonia carries out large quantities of water vapor, resulting in concentration of the ammonium caseinate in the remaining slurry liquid and thus protecting the plaster against the setting which can occur through failure of the drier or existence of local cool areas therein. (3) the considerations in (2) also permit drying at a minimal temperature, for example, at 200 F. as in Example 1 or even at 190 F. (88 C.), since even if normal variations in drier operation occasionally occur to result in somewhat lower temperatures, the partially dry Web is protected against setting which can occur below about 90 C. in the absence of a set-inhibitor. Applicants high concentration of ammonia also causes the plaster to become more dense during the heating of the slurry, a desirable effect that does not occur where minimal concentrations of ammonia are used as in U.S. Patent 2,557,083.

The time and temperature required for satisfactory drying of the slurry-coated Web, will depend to some extent upon the amount and nature of additional binders present and, among other factors, particularly on the characteristics of the drier, for example, the rate and distribution of circulating air, whether air circulate over both surfaces of the Web, amount of heat transfer by convection and radiation, etc. It is, of course, not necessary in applicants process to dry at a temperature above the boiling-point of Water. Water Will evaporate Whenever its vapor pressure is greater than the partial pressure of water vapor in cont-act with it, although increased air circulation and higher temperatures accelerate the drying. Usually the temperature will be above about 90 C. since above this temperature plaster will not set at atmospheric pressure. However, too long heating at too high temperature can form anhydrite, which is undesirable since this compound is often slow to hydrate as the bandage is wet for use. If the dried bandage, after standing for 3 to 4 hours protected from moisture, does not feel gritty when wet for use, due to prior setting by moisture contained therein, the drying is satisfactory and need not and preferably should not be carried substantially further. Furthermore, whether the bandage is dry can generally be determined by visual inspection, since any wet areas show a slightly dark (grayish) appearance by contrast against the usually nearly pure white dry areas. Thus visual inspection followed by later examination for grittiness in the subsequently wetted bandage, are the practical criteria for satisfactory drying.

The plaster of Paris used in Example 1 is of the type described in Us. Patent 1,901,051 and British Patent 5 63,019. Such plaster is sometimes called alpha-gypsum or, better, alpha-hemihydrate of calcium sulfate. It is a high density, low consistency, finely-ground plaster. (Consistency or Pouring Consistency, determined by the patty method of US Gypsum Company Bulletin I.G.L. No. 19, is the minimum number of cubic centimeters of water required to be mixed with 100 grams of plaster in order that the unset slurry will just pour from the lip of a cup.) The consistency is related to the density and particle size of the plaster-being, within limits, lower the higher the density and the larger the size. While the present methods are applicable to low-density plaster, the high density product is preferred because it rquires less slurry liquid and gives a more compact, stronger cast. In order to make a bandage thatt will give a high-strength -castespecially at the important early stages, for example, /2 to 1 hr. after the bandage is wet preparatory to making the cast-applicant has discovered it is necessary to use plaster that is finely-ground and also of proper particle-size distribution. The plaster of Example 1 had a consistency'of 38 to 42. The particles were 0.1% on 30 mesh National Bureau of Standards scree-n (opening 590 microns), 97% through 100 mesh (opening 149 microns), 82% through 325 mesh (opening 44 microns). Plaster of 33 to 34 consistency; 99.9% through 200 mesh (opening 74 microns) and 95% through 325 mesh, did not give nearly as high /2 hr.

cast strength as the plaster of Example 1, in spite of yielding a higher compressive strength in a briquette of pure plaster. Thus the distribution of particle sizes is very important in addition to fineness and high density, in the case of a plaster of Paris bandage as contrasted to the case of pure plaster casts. This is totally unexpected since it has been generally thought that high early strength in a cast made from a plaster of Paris bandage required high density alone. Applicant also finds it is necessary to use a plaster with a pH of 6 to 7. Alkaline plaster is unsuitable. In general, the preference is for a plaster with consistency not above about 45 and with particles 99.5% to 100% of which pass a 100 mesh screen (otherwise a grittiness is detectable in the wet bandage), fro-m 50 to 90% through 325 mesh and 10 to about evenly distributed between 100 and 325 mesh. The potassium sulfate should also be at least 995% through 100 mesh, else any undissolved crystals will impa-rt grittiness to the wet bandage.

Large quantities of applicants bandage have been made under plant conditions and widelytested by a large independent marketer and shown to have characteristics, as described hereinbefore, superior to those of bandages available in the trade.

All percentages and proportions given hereinbefore are by weight, w

What is claimed is:

1. A process for making a settable, dry plaster of Paris bandage which comprises in succession the steps of (1) making a slurry of powdered plaster of Paris in an aqueous solution containing from 4 to 20% by weight of NH and 0.1 to 1% by weight of casein dissolved therein, while said slurry remains substantially unset; (2) impregnating the product of step (1) into crinoline in amount to yield a dry weight of from 205 to 255 grams per 4 inch by 5 yard strip thereof; (3) heating the product of step (2) at a temperature and for a time sufficient to substantially hydrolyze the dissolved ammonium caseinate to NH and casein and to evaporate substantially all of the NH; and water so that a final, dry, settable bandage is obtained in which the set is not substantially retarded, but not heating substantially beyond the point where said dry bandage shows no substantial grittiness when wet with water after standing protected from moisture for 3 to 4 hours.

2. The process of claim 1 wherein to the said slurry of step (2) is further added from 0.2 to 2% of the weight of said plaster of Paris of a material selected from the class consisting of cooked starch, dextrin, methyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl ethyl cellulose, and mixtures thereof; and wherein the said crinoline of step (2) is replaced by surgical gauze.

3. The process of claim 2 wherein there is further added 'to the said slurry :of step (2) from 0.5 to 2.5% by weight of finely-divided potassium sulfate, based on the weight of said plaster of Paris.

4. A set retarded plaster of Paris mix comprising a major portion of plaster of Paris and water and a minor portion of set-inhibitor comprising substantially 4 to 20% by weight of dissolved NH based on the total weight of NH and water, and 0.1 to 1% by weight of casein based on the total'weight of NH and water; said casein being substantially undegraded and being dissolved in the NH and Water.

5. The product of claim 4 wherein the said casein is from 0.1 to 1% of the weight of said plaster of Paris.

6. The product of claim 4 wherein the said mix further contains from 0.2 to 2% of the weight of said plaster of Paris of a material selected from the class consisting of cooked starch, dextrin, methyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl ethyl cellulose, and mixtures thereof.

7. The product of claim 6 wherein the said mix further contains from 0.5 to 3% of potassium sulfate based on the weight of plaster of Paris, at least about 99.5% by weight of the potassium sulfate being of a size to pass a mesh National Bureau of Standards screen.

References Cited by the Examiner UNITED STATES PATENTS 1,638,001 8/1927 Brookby 106112 1,726,403 8/1929 Mathey 1289l 2,557,083 6/1951 Ebcrl 12891 2,842,120 7/1958 Foglia 12891 2,842,138 7/1958 Billings et al. 12891 3,043,298 7/ 1962 Brickrnan et al. l28-91 3,191,597 6/1965 Smith l06-112 TOBIAS E. LEVOW, Primary Examiner.

SAMUEL H. BLECH, Examiner.

S. E, MQTT, Assistant Examiner, 

1. A PROCESS FOR MAKING A SETTABLE, DRY PLASTER OF PARIS BANDAGE WHICH COMPRISES IN SUCCESSION THE STEPS OF (1) MAKING A SLURRY OF POWDERED PLASTER OF PARIS IN AN AQUEOUS SOLUTION CONTAINING FROM 4 TO 20% BY WEIGHT OF NH3, AND 0.1 TO 1% BY WEIGHT OF CASEIN DISSOLVED THEREIN, WHILE SAID SLURRY REMAINS SUBSTANTIALLY UNSET; (2) IMPREGNATING THE PRODUCT OF STEP (1) INTO CRINOLINE IN AMOUNT TO YIELD A DRY WEIGH OF FROM 205 TO 255 GRAMS PER 4 INCH BY 5 YARD STRIP THEREOF; (3) HEATING THE PRODUCT OF STEP (2) AT A TEMPERATURE AND FOR A TIME SUFFICIENT TO SUBSTANTIALLY HYDROLYZE THE DISSOLVED AMMONIUM CASEINATE TO NH3 AND CASEIN AND TO EVAPORATE SUBSTANTIALLY ALL OF THE NH3 AND WATER SO THAT A FINAL, DEY, SETTABLE BANDAGE IS OBTAINE IN WHICH THE SET IS NOT SUBSTANTIALLY RETARDED, BUT NOT HEATING SUBSTANTIALLY BEYOND THE POINT WHERE SAID DRY BANDAGE SHOWS NO SUBSTANTIAL GIRTTINESS WHEN WET WITH WATER AFTER STANDING PROTECTED FROM MOISTURE FOR 3 TO 4 HOURS. 